Blue-shifted and red-shifted hydrogen bonds:: Theoretical study of the CH3CHO•••HNO complexes

The blue-shifted and red-shifted H-bonds have been studied in complexes CH3CHO center dot center dot center dot HNO. At the MP2/6-31G(d), MP2/6-31+G(d,p) MP2/6311++G(d,p), B3LYP/6-31G(d), B3LYP/6-31+G(d,p) and B3LYP/6-311++G(d,p) levels, the geometric structures and vibrational frequencies of complexes CH3CHO center dot center dot center dot HNO are calculated by both standard and U-corrected methods, respectively. Complex A exhibits simultaneously red-shifted C-H center dot center dot center dot O and blue-shifted N-H center dot center dot center dot O H-bonds. Complex B possesses simultaneously two blue-shifted H-bonds: C-H center dot center dot center dot O and N-H center dot center dot center dot O. From NBO analysis, it becomes evident that the red-shifted C-H center dot center dot center dot O Hbond can be explained on the basis of the two opposite effects: hyperconjugation and rehybridization. The blue-shifted C-H center dot center dot O H-bond is a result of conjunct C-H bond strengthening effects of the hyperconjugation and the rehybrictization due to existence of the significant electron density redistribution effect. For the blue-shifted N-H center dot center dot center dot O H-bonds, the hyperconjugation is inhibited due to existence of the electron density redistribution effect. The large blue shift of the N-H stretching frequency is observed because the rehybridization dominates the hyperconjugation.